The present invention relates to mechanical kinetic vacuum pumps particularly pumps having rotors made of a light metal alloy by powder metallurgy.
By definition gaseous ring vacuum pumps, turbo vacuum pumps (axial, radial) and molecular/turbomolecular pumps belong to the class of mechanical kinetic vacuum pumps. They are capable of mechanically transporting within the molecular flow range (pressures below 10−3 mbar) the gas particles which are to be pumped. Moreover, molecular pumps are also capable of pumping gases within the Knudsen flow range (10−3 to 1 mbar). Preferably employed mechanical kinetic vacuum pumps frequently offer a turbomolecular pumping stage and a downstream molecular pumping stage (compound or hybrid pump), since such a pump is capable of compressing gases up in to the viscous flow range.
Turbomolecular vacuum pumps and compound pumps are employed in production processes of the semiconductor industry. The processes which are applied—etching, coating and the like—are only performed in a vacuum. The mentioned vacuum pumps have the task of evacuating the vacuum chambers before starting the processes and to maintain during the course of the process the desired low pressures.
Turbomolecular vacuum pumps are operated at high rotational speeds (up to 100,000 rpm). For reasons of rotor dynamics the rotors consist of a light metal, commonly an aluminium alloy produced by melt metallurgy, such as casting. The alloy is so adjusted that the rotors offer a high degree of resistance to heat and creep. Creep reduces with increasing rotor temperatures. In the instance of the aluminium alloys employed to date, the creep is acceptable, provided rotor temperatures of 120° C. are not exceeded.
Whilst performing the semiconductor processes, the semiconductor components located within the vacuum chamber attain increased temperatures. This results in an increase in temperature affecting the gases to be conveyed by the vacuum pumps. These gases effect in particular a temperature increase of the rotors in the connected vacuum pumps. Said temperature increase impairs the creep characteristics mentioned, i.e., the rotor temperatures can rise to a temperature at which unacceptable creep starts occurring.
Cooling the rotors of a molecular or turbomolecular vacuum pump is difficult. On the one hand the rotors operate in a vacuum so that no heat is dissipated via the pumped and anyhow hot gases. If the rotors are magnetically suspended, their bearing components will not make contact. Heat dissipation via the magnetic bearings is thus also not effective. If mechanical bearings are provided, the heat of the rotors may be dissipated via the bearings. However, this means of dissipating heat has tight limitations. On the one hand the surfaces of rotor and stator in contact via the rolling bodies are restricted to the almost point shaped contact surfaces of the rolling bodies in their bearing rings. Moreover, due to the presence of a lubricant, the bearings must not attain high temperatures. Also operation of the mechanical bearings themselves incurs the generation of heat. Finally, in general, the drive motor of the pump is a component of the stator and located in the vicinity of the bearings. During those phases where it is operated under a load, it itself forms a source of heat. In this instance a partial transfer of heat between rotor and stator is possible via the gas owing to the increased density. Dissipation of significant quantities of heat via the mechanical bearings would only be possible in the instance of intense cooling of the bearing section on the stator side.
From JP-U-3034699 it is known to coat the active pumping surfaces of a mechanical kinetic vacuum pump in part with a high heat emission coating. Measures of this kind are involved and thus costly.
From the U.S. Pat. No. 6,095,754, a mechanical kinetic vacuum pump for deployment in semiconductor processes is known. It is designed by way of a turbomolecular vacuum pump. In order to attain the target of reducing the duration of semiconductor processes, the task is posed to improve the pumping capacity of the pump. In doing so, the size of the pump is not to be changed. For the purpose of solving this task the employment of a stronger material suited for higher temperatures is proposed preferably for the rotor, specifically a material consisting of a metal as the base material and non-metal additives, like ceramics, for the purpose of reinforcing the base material. Said stronger material allows an increase in rotor speed in order to attain through an increased thermal load a subsequent increase in pumping capacity without changing the size of the pump. The metal cutting process to which the proposed materials are subjected incurs problems owing to the increased share in hard material particles. Rotors for turbomolecular vacuum pumps including the multitude of their blades are commonly turned on a lathe or milled from solid material. The percentage of chips produced in the manufacture of a rotor amounts up to 80%. Thus the manufacture of rotors made of the proposed material is involved and costly.
It is the task of the present invention to increase the resistance to heat and creep for a friction vacuum pump of the aforementioned kind.